Which Python to use is not something you configure in IPython. IPython is just a package, so you must install it separately for each Python in which you want it to be importable. If your distro does not have a package for IPython on Python 3, you can always easy/pip install it:

pip-3.3 install ipython

(Note: you may need to install pip first, you may need to prefix with sudo, or do --user, etc.)